The present invention relates to a process for cleaning coal of fine particle size. More particularly, it is concerned with a new and improved process for mechanically removing unwanted contaminant particles from fine coal and dewatering the cleaned coal sufficiently to permit stockpiling.
In the cleaning, upgrading or cencentration of coal for commercial use as fuel and the like, it is necessary to remove rock and other contaminant particles that are mined with the coal. The rock and other impurities are generally unavoidable in thin-seam mining where the coal stratum is of limited thickness. In such cases it is very difficult to remove the coal without also taking minor amounts of host rock which encloses the coal seam. As will be appreciated, the least possible amount of rock is preferred in order to provide the highest possible quantity of combustible material and least amount of unwanted residue. Another typical impurity is sulfur in the form of pyrite (iron sulfide). It undesirably generates harmful sulfur-containing gases which decrease the efficiency of combustion and provide sulfur oxide compounds contributing to unwanted air pollution levels.
The treatment of coal for the purpose of removing these impurities conventionally involves the preliminary step of classifying the crushed coal into two size fractions. The large or coarse fraction conventionally exhibits a size range of from about 3 inches to about 1/4 inch, while the second or "fine" fraction comprises all of the mined material smaller than 1/4 inch. This classification is readily achieved by passing the crushed coal across a sizing screen deck. A plurality of water sprays are used to assist in the separation of the fine coal from the coarser material.
The coarse coal can be treated to remove the rock and impurities of high specific gravity by means of a heavy media separation technique. In that process the coarse coal fraction is placed in a tank filled with a liquid having a specific gravity slightly higher than coal. The coal tends to float on the surface of the liquid while the impurities that exhibit a substantially higher specific gravity sink to the bottom of the tank and are removed separately, as waste material. The coarse coal is then easily dewatered on conventional dewatering screens prior to stockpiling.
Unfortunately, the heavy media technique used for coarse coal is not effective for cleaning the fine materials, that is, those having a particle size of about 1/4 inch and less. An appreciable amount of the small impurity particles possess insufficient mass to sink within the dense liquid media, and become permanently suspended within the liquid, thereby preventing their separating from the coal particles of equal size.
Various chemical and mechanical processes have been used for cleaning the fine coal material. These have included chemical flotation, shaking tables, heavy media cyclones, water only cyclones and air separation. The cleaned material is then centrifuged to dewater the material for storage. Among these techniques, the most commonly used system has been the use of shaking table separators combined with dewatering centrifuges. The vibratory motion of the shaking tables have the disadvantages of poor efficiency below 48-mesh and high energy consumption. The shaking tables also require large plant floor space per ton processed and have a relatively high initial cost. The prior systems also suffer from the high maintenance cost of centrifugal drying apparatus and the tendency to grind the particles during the centrifuging operation, resulting in an ultrafine material, much of which is suspended within and discarded with the process waste effluent.
Accordingly, it is an object of the present invention to provide a new and improved process for the wet concentration and dewatering of fine coal. Included in this object is the provision for a new and improved wet mchanical method that utilizes a three-step technique for sequentially and selectively removing impurity fractions, and efficiently dewatering the concentrated fine coal product to an extent sufficient to permit stockpiling, shipment and sale.
Another object of the present invention is to provide a wet mechanical process for the concentration of fine coal that provides improved coal recovery.
Yet another object of the present invention is to provide a process of the type described that achieves beneficiation of fine coal at significantly lower investment cost in both plant and equipment and substantially lower maintenance cost and power consumption.
Still another object of the present invention is to provide a mechanical process for the wet concentration of fine coal that includes the benefits inherent in the utilization of a gravity flow centrifugal separation. Coupled with this is the utilization of a high efficiency dewatering technique that provides for the complete removal of substantially all of the free excess water, thereby delivering a cleaned and dewatered coal product to a stockpile location without the need for subsequent dewatering operations.
A further object of the present invention is to provide a process that concentrates and fractionates the raw feed stock to clean and upgrade the fine coal followed by dewatering without loss of the very fine coal particles. Included in this object is the provision for a process of the type described resulting in a deslimed fine coal fraction substantially free of ultra-fine particles while obviating losses resulting from the grinding of particles in a centrifugal drying operation.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a process for cleaning fine coal that comprises the steps of forming a water slurry of fine coal and its associated contaminant particles wherein all particles have a particle size of less than about 10 mm.; feeding the slurry to a spiral gravity concentrator so that as the slurry flows downwardly along the spiral, the heavier contaminants having a particle size greater than about 0.1 mm. concentrated in a band separate from the remainder of the slurry for subsequent removal and discarding; feeding the remainder of the coal slurry to a hydrocyclone separator and regulating the discharge from the descending vortex thereof to provide controlled accumulation at the descending vortex of a coal particle fraction having a size greater than 0.1 mm; retaining in suspension and discarding all of the ultra-fine silt material reporting to the ascending vortex of the hydrocyclone and dewatering the collected coal fraction treated in both the spiral concentrator and hydrocyclone separator sufficiently to permit stockpiling of the cleaned coal.
A better understanding of this invention will be obtained from the following detailed description and the accompanying drawing wherein the several steps of the process and the relation of one or more of such steps with respect to each of the others are described together with the product thereof and the features, properties, and relation of elements described and exemplified herein.